The present invention generally relates to satellite communication systems, and more particularly to a multi-beam satellite system employing onboard switching for the purpose of increasing the up path and the down path antenna gains.
It is sometimes desirable to increase the effective radiated power from a satellite in a given direction toward the ground in order to increase the channel capacity in that direction, and/or to permit the use of a smaller earth station antenna, and/or a less costly earth station receiver (one with a higher noise level). The need for such an approach can be appreciated when one considers present designs wherein a number of satellite-born transponders, each on a different frequency, connect to a single antenna illuminating a given area A.sub.0 as illustrated in FIG. 1 of the drawings. This system is very flexible in that all signals from all transponders can be received anywhere in the area A.sub.0. Thus, the outputs of all transponders, or some of these down to a single one can be addressed to a given station on the ground, and this number of transponders can be varied with time. The disadvantage of this system lies in that the power of all transponders is constantly illuminating the entire area A.sub.0 even though the signals from one or more transponders may be addressed to only a single station in the entire area. In the limiting case, it would be desirable for each satellite transponder to illuminate only the station addressed.
Similarly, the use of higher gain receive antennas on the satellite permits the use of greater channel capacity from a given earth station, and/or a smaller earth station antenna, and/or a lower powered transmitting amplifier in the earth station.
The present approach to improving on this situation is to narrow the satellite antenna beams both in the earth-to-satellite and also the satellite-to-earth directions as shown in FIG. 2. In this arrangement, four separate up beams and four separate down beams are shown, although a greater or smaller number may be used. Each set of satellite receiving antennas 11, 12, 13 and 14 are directed at the corresponding areas A.sub.1, A.sub.2, A.sub.3 and A.sub.4 within the larger area A.sub.0 as shown in FIG. 1. The receiving antennas 11, 12, 13 and 14 are permanently connected to the input amplifiers 15, 16, 17 and 18, respectively. The switch 19 connects each input amplifier to a different output amplifier 21, 22, 23 and 24 on a sequential basis. Typically, a mixer and a suitable oscillator are provided for each chain so as to produce a common IF for switching. An output mixer and oscillator per chain may also be added as shown in the figure. This is a classic arrangement used in Intelsat I where down conversion was made from 6GHz to 60MHz followed by up conversion to 4GHz. Each output amplifier 21, 22, 23 and 24 connects to only one transmitting antenna 25, 26, 27 and 28, respectively, and each of these transmitting antennas is directed to a different area on the ground corresponding to the areas A.sub.1, A.sub.2, A.sub.3 and A.sub.4 shown in FIG. 1. The arrangement shown in FIG. 2 assumes a time division multiplex system with the various stations properly synchronized with each other, and such arrangements have been described previously. A shortcoming of such systems is their relative inflexibility in that only one output amplifier is associated with each beam. It is possible to make one given amplifier of higher power than another or to connect more than one amplifier to a given beam, since each amplifier is assumed to cover a different frequency band from the others, as a means of increasing the channel capacity in a given direction; however, this presupposes a known traffic pattern.